1. Field of the Invention
The present invention relates to a driving method usable to modulate a light source device for optical transmission according to an analog or digital signal and achieve the transmission, such as high-density wavelength division multiplexing (WDM) optical communications, while suppressing dynamic wavelength variation during fast modulations. The present invention also relates to a semiconductor light source device using such a driving method, and optical communication method and system using such a driving method or a device.
2. Description of the Related Art
As an oscillation polarization-mode selective or switchable dynamic single mode semiconductor laser, the following device has been developed and proposed (see, for example, Japanese Patent Laid-Open No. 2-159781 (1990)). The oscillation polarization mode of the polarization switchable laser is changed by a digital signal which is produced by superimposing a minute-amplitude digital signal on a bias injection current. The device is a distributed feedback (DFB) laser in which a distributed reflector, formed of a grating, is introduced into a semiconductor laser resonator cavity, and its characteristic of wavelength selectivity is utilized therein. In the device, a strain is introduced into an active layer of a quantum well structure, or the Bragg wavelength is located at a position shorter than a peak wavelength of its gain spectrum, so that gains for the transverse electric (TE) mode and the transverse magnetic (TM) mode are approximately equal to each other for light at wavelengths close to an oscillation wavelength, under a current injection condition near its oscillation threshold state. Further, a plurality of electrodes are arranged and currents can be unevenly injected through those electrodes. An equivalent refractive index in the cavity is nonuniformly distributed by such uneven current injection, and the laser oscillates in either the TE mode or the TM mode and at a wavelength which satisfies a phase matching condition and shows a minimum threshold gain. When the balance of the uneven current injection is slightly changed to vary a competitive relation of the phase condition between the TE mode and the TM mode, the polarization mode and the wavelength of the laser can be switched.
As a driving method of that device, a TE/TM switching method has been proposed in which the above minute-amplitude digital signal is superimposed on a bias current injected through one electrode of a two-electrode DFB semiconductor laser. This is the technique according to which bias points of currents injected through the two electrodes are appropriately set, the polarization-modulated output light from the laser is transmitted through a polarizer to be converted to intensity-modulated light, and TE-mode polarized light can be thus obtained with a large extinction ratio.
In the above-discussed digital polarization modulation system, however, when transmission of an analog signal, such as a video signal, is to be performed, the analog signal needs to be converted to a digital signal by an analog-to-digital converter and the signal is sent thereafter. Hence the number of components of the system increases in the analog video transmission. Further, the system could not cope with the expansion of a transmission capacity aimed by a multi-value digital transmission system.
An object of the present invention is to provide a driving method or apparatus for driving a polarization switchable laser to generate an intensity-modulated signal (typically, an intensity-modulated analog signal or a multi-value intensity-modulated digital signal), and a light source apparatus for performing optical transmission using such a driving method. Another object of the present invention is to provide an optical communication system or method for performing transmission of an intensity-modulated signal using such a light source apparatus, and a multiplexing optical communication system or method for performing transmission of multiplexed intensity-modulated signals using such a light source apparatus.
The present invention is directed to a driving method or apparatus for driving a semiconductor laser as a light source for optical transmission, wherein a semiconductor laser is used, which is provided with a structure in which oscillation light of the laser can be switched between two different polarization modes by modulating a current injected into a portion of a waveguide of the laser. The laser has characteristics that light in both the two modes is emitted within a polarization-mode switching range of the current and that intensities of the light in both the two different polarization modes respectively vary in accordance with an amount of the current within the polarization-mode switching range. The current injected into the portion of the waveguide is urged to a bias point current in the switching range, and a modulation signal current is superimposed on the bias point current such that the emitted light in at least one of the two modes is intensity-modulated in accordance with an amount of the modulation signal current. The above modulation signal current is an analog-signal modulation current, an analog-signal modulation current generated from a digital signal by a-modulation unit, a multi-value digital-signal modulation current generated from a digital signal by a modulation unit, or the like.
The principle of a driving method or apparatus of the present invention will be described referring to FIG. 1 showing an example of a curve of I (a current injected into the portion of the waveguide) vs. L (light output) for TE-mode light and TM-mode light from the laser during DC driving. The curve shows outputs of the TE-mode light and TM-mode light appearing when a constant current is injected into an electrode on, a light emission side of the laser and a current injected into another electrode on a modulation side of the laser is changed. Here, the semiconductor laser has characteristics that light in both polarization modes is emitted within the switching range of the current and that intensities of the light in both polarization modes vary substantially linearly in accordance with an amount of the current within the switching range. Where the polarization switchable laser is a single mode laser, such as a DFB laser, both of the TE-mode light and TM-mode light oscillate in a single longitudinal mode at Bragg wavelengths (differing between the TE mode and the TM-mode) determined from the grating of the DFB laser or the like.
There exists a bias state in which the TE-mode light and TM-mode light coexist; near a switching point in the I-L curve (or in the switching range). The width of the current in the bias state (i.e., the width of the switching range) is indicated by xcex94 Isw. When a modulation signal (here, an analog modulation signal is shown) is applied with an amplitude smaller than xcex94 Isw, the TE-mode light is intensity-modulated with a large amplitude due to the characteristic that the I-L curve steeply rises in the switching range. Where a sign-inverted modulation signal is applied, desirably-modulated TM-mode light can be obtained.
In the above discussion, the example is selected which has the switching range wherein TM-mode light is switched over to TE-mode light as the bias current injected into the portion of the waveguide increases. However, when the I-L curve is selected which has the switching range wherein TE-mode light is switched over to TM-mode light as the bias current increases, intensity-modulated light in the TM-mode can be naturally obtained. In this case, the relation between TE-mode light and TM-mode light in FIG. 1 is reversed by adjusting bias currents injected into respective waveguides.
More specifically, the following configurations can be adopted.
The semiconductor laser may be comprised of a distributed feedback semiconductor laser wherein a waveguide and a diffraction grating formed near the waveguide are arranged, the waveguide includes an active layer, the active layer is formed of a multiple quantum well structure into which a tensile strain is introduced, and a heavy-hole ground level in a quantum well of the multiple quantum well structure is nearly equal to a light-hole ground level in the quantum well of the multiple quantum well structure, or the light-hole ground level is closer to an electron ground level than the heavy-hole ground level is. Thus, gains for the TE mode and the TM mode can be made nearly equal in the DFB laser, and the above switching can be achieved by such a polarization switchable laser.
Further, the semiconductor laser may be comprised of a structure in which there are arranged at least two electrodes in a cavity direction of the laser, there are arranged a plurality of waveguide portions, which constitute the cavity, corresponding to those electrodes such that currents can be independently injected into those waveguide portions, the respective waveguide portions include active layers with a plurality of quantum wells which respectively include at least one non-strained or compressively-strained quantum well and at least one tensile-strained quantum well, band gaps between ground levels in those quantum wells are set nearly equal to each other, and combinations of the quantum wells in those waveguide portions are different from each other. Currents can be independently injected into the thus-constructed waveguide portions, and their injection current dependencies are different from each other, so that round-trip gains for the TE mode and the TM mode can be made close to each other. As a result, the bias state of injection currents, in which oscillation modes compete with each other, can be established. The above modulation can be performed by the above driving method in the laser urged to such a bias state. Since the active layer in each waveguide portion includes at least one well for contributing to the TE-mode gain and at least one well for contributing to the TM-mode gain, each active layer has gains for both polarization modes. Further, combinations of those wells are different between those waveguide portions, so that injection current dependencies of gain spectra of those active layers differ from each other. Since each waveguide portion has gains for both modes, fluctuation of the light intensity distribution in the cavity is small during polarization switching. Thus, switching characteristic that wavelength fluctuation due to the hole burning effect is small can be obtained.
The semiconductor laser with suppressed hole burning effect may be comprised of a distributed feedback semiconductor laser wherein a waveguide with an active layer and a diffraction grating formed near the waveguide are arranged. The polarization switching is effected between the TE mode and the TM mode at Bragg wavelengths of the diffraction grating (i.e., between single longitudinal modes), and the switching can be stably achieved by a minute current.
In the driving method or apparatus of the semiconductor laser, an amount of combined current of the modulation signal current and the bias point current is preferably regulated well within the switching range in which light in both polarization modes is emitted. Hence, the intensity modulation with little non-linearity and preferable linearity (i.e, small or little distortion from the original signal) can be achieved.
The driving apparatus may further include a unit for separating the intensity-modulated light in the two different polarization modes emitted by the semiconductor laser from each other, a unit for coupling the separated light in one of the two different polarization modes to a light transmission line, a unit for converting the separated light in the other of the two different polarization modes to an electric signal, and a unit for controlling the current injected into the semiconductor laser based on the electric signal such that a modulation condition of the light coupled to the light transmission line can be stabilized. Hence, the current bias state can be monitored by using the light in the other mode not used for transmission, and the bias point can be stabilized to assure preferable linearity of the signal.
The controlling unit may includes a low pass filter for transmitting the electric signal therethrough, a differential amplifier for comparing a signal output from the low pass filter with a reference voltage to supply its resultant output, and a proportional-plus-integral circuit for feeding back the resultant output to a DC current source for driving the semiconductor laser at an appropriate feedback rate. The bias point can be stably controlled by a simple structure.
A light source apparatus for optical transmission for achieving the object of the present invention includes the above driving apparatus for driving the semiconductor laser as a light source for optical transmission using the above driving method, and a polarization-mode selecting unit for selecting light in one of the two different polarization modes from light emitted from the semiconductor laser.
These advantages and others will be more readily understood in connection with the following detailed description of the preferred embodiments in conjunction with the drawings.